1. Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine and a facsimile machine which performs image forming with an electrographic system, and in particular, related to an image forming apparatus to peel a sheet from a fixing portion by ejecting compressed air toward a sheet top end after fixing.
2. Description of the Related Art
In general, an image forming apparatus to form an image with the electrographic system is provided with a fixing apparatus as a fixing portion including a heat roller as a fixing rotating member with a heat source and a pressure roller as a pressing rotating member to be pressure-contacted to the heat roller. A sheet having a non-fixed toner image formed is conveyed to a nip portion of the heat roller and the pressure roller. Then, the fixing apparatus melts non-fixed toner by applying heat and pressure to the sheet which passes through the nip portion, so that the toner image is fixed on the sheet.
When a toner image is fixed by applying heat and pressure to a sheet, although rare but there has been a case that the sheet is twisted around the heat roller due to melted toner. In order to solve this problem, a fixing apparatus as described above has been provided with a separation claw contacted to the heat roller. With the separation claw, the sheet is easily peeled from the heat roller when image density is low or when distance from a sheet edge to an image (hereinafter called white space) is spaced.
However, with a small grammage sheet (particularly, coated paper), for example, sticking force of melted toner to the heat roller becomes large in the case of high density color image or less white space image, so that the separation claw is hard to enter between the sheet and the heat roller. Accordingly, there has been a case that the separation claw hurts the edge of the sheet. Further, since the separation claw is contacted to the melted toner image, there has been a problem that the toner image is apt to be damaged. Furthermore, since the separation claw is contacted to the heat roller, there has been a case that the heat roller is scratched and poor imaging occurs due to transfer of the scratch to the melted toner.
As a separation method without damaging an image to be fixed on a sheet and the heat roller, a separation method having a nozzle which is arranged between the heat roller and a peeling guide plate arranged at the vicinity thereof and ejecting compressed air from the nozzle has been proposed. With this separation method, the sheet top end stuck to the heat roller is separated from the heat roller by ejecting compressed air and the entire sheet is peeled from the heat roller by the peeling guide plate thereafter.
With the separation method utilizing compressed air, pulse-like compressed air is blown to a surface of the heat roller from the nozzle which is constituted with a pipe-shaped introduction path and an ejection port arranged at the top end thereof. Accordingly, when the compressed air ejected from the nozzle is ejected to a part of the heat roller several times, there has been a case that surface temperature of the heat roller is decreased in part. When the surface temperature of the heat roller is decreased, there has been a case that poor imaging such as gloss unevenness and poor fixing is caused on a fixed image to be obtained.
Accordingly, there has been a proposal to prevent poor imaging by increasing temperature of ejection air while heating the compressed air to be ejected to prevent temperature decrease of the heat roller (see Japanese Patent Application Laid-Open No. 2005-157179).
However, with the abovementioned method to heat compressed air, a heater for heating air is required and power is consumed to operate the heater. Therefore, there has been a problem of cost increase.
The present invention provides an image forming apparatus capable of preventing poor imaging caused by temperature decrease of a fixing rotating member due to ejected air and capable of reducing cost and power consumption.
According to the invention, an image forming apparatus includes:                a transfer portion which transfers a toner image to a sheet;        a fixing portion which includes a fixing rotating member having a heat source and a pressing rotating member being pressure-contacted to the fixing rotating member and which fixes the toner image on the sheet at a nip portion of the fixing rotating member and the pressing rotating member; and        an air ejecting portion which is capable of ejecting air toward a gap between the top end of the sheet having passed through the nip portion and the circumference of the fixing rotating member and which includes a plurality of air ejection positions arranged in the width direction being perpendicular to a sheet conveying direction;        wherein a control portion controls the air ejecting portion so as to selectively eject air.        